Cylinder problem

I am unable to completely isolate the hot water supply to the taps by shutting off the cold feed to the cylinder as normal (valve is OK) . Therefore have assumed that water is leaking into the cylinder from the primary circuit, i.e. the cylinder is knackered.

Does this sound a reasonable assumption, and if so what problems should I look out for when replacing the cylinder. (Some of the fittings i.e. the isolating valves are imperial).

Any advice welcomed.

If the means of isolation is a gate valve its more likely that thats passing even if it feels like its working when you turn it.

Flippin hot water gate valves are rubbish from the day they are fitted, dont be tempted to tighten the valve with a wrench as they are prone to snapping off in the off position, then it is a right pain to replace.

i think alll the fittings on the cyclinders are imperial 1 inch but the fitting are 1 inch to 22 comp so no probs there or 3/4 to 22 or 1/2 to 15 etc

You wish to change cylinder because you are unable to shut off hot water
Before you go spending money on a new cylinder you should check that there is a problem
Tie up your ball valve to a piece of clean wood resting across the HW cistern so cutting off supply
Run your hot water tap until cistern empties then see if water is still coming through your tap which will prove if water is leaking from your coil or not.
You will also see heating cistern keep refilling if this is the case.
Leafy and PVMan have probably diagnosed your problem correctly as to a faulty gate valve which should be replaced with a full bore isolater